The transparent colour is BLACK, which is colour 0x00. When this colour is selected for a derived category, the effective colour is that of the base category. To force black, use colour 0x10 which is distinct from black but has the same hardware settings (i.e. it looks identical).
When the selected line ends are 'unknown' (indicated by a red circle with a red diagonal), the effective line ends are those of the base category.
When the selected line thickness is 'unknown' (indicated by a red circle with a red diagonal), the effective line thickness is that of the base category.
Objects belonging to a particular category can be added to a group by using the gather facility. To invoke this facility, an object belonging to the category we are interested in must be selected at a point which lies inside the bounding rectangle of the group we want the objects to be added to. The objects menu must be brought up by using the right mouse button to do the selection. Chose the 'gather' option in the objects menu, and IPAD will add all objects that cut (or are completely enclosed by) the groups bounding rectangle. If there is any ambiguity about which group to add the objects to, then the operation will be aborted and neither of the groups will be modified.
When using the gather facility, the first category of the objects category list is the one used for the selection.
When modifying the category of a group of objects, the category list of the group will be the combined categories of all the objects contained by the group. Adding a category to the group, will cause the category to be added to all objects in the group. Removing a category from the group, will cause the category to be removed from all objects in the group.
When grouping by category, it is often useful to start with an empty group. An empty group can be simply created by grouping a small blank area of the drawing. The empty group can then be moved to the required position and resized just like any other rectangle, without effecting any other objects.
Examples of using categories are given in the drawings 'cat*_eg.ipd'
Where a large number of complex changes need to be made to the category model of a drawing, it is possible to export the drawing, edit it using a text editor or some other filtering program and then load it again.